1. Field of the Invention
The present invention relates to the field of dual technology sensors. More particularly, the invention concerns dual technology sensors using microwave and infrared sensing elements.
2. Description of the Prior Art
Dual technology sensors are increasingly being used for intruder detection and the like. A particularly popular combination of technologies involves a microwave section and a passive infrared section.
As is well known, in a dual technology sensor an alarm signal will be produced only when both of the constituent technologies detects an intruder. Thus the sensor will be rendered useless if one of the two technologies should become inoperative. It is clearly desirable, then, that the two detectors should periodically or intermittently test themselves and, if a malfunction is detected, generate a fault indication.
It is common for the microwave section of a dual technology sensor to use an oscillator operating in a pulsed mode. The interval between successive pulses of microwave energy is controlled to give the desired power output (and, thus, detection range). Typically, each power pulse includes a large number of cycles of the oscillator output and occupies only a small fraction of the interpulse period. An example of a dual technology sensor of this type is the Twintec 1080 manufactured by Scantronic Limited.